The invention relates to an electric lamp comprising a glass lamp vessel sealed in a vacuum-tight manner and having a largest diameter. The lamp vessel has a translucent wall portion and a wall portion which is mirror-coated on its inner surface with an aluminum layer, this mirror-coated wall portion having a boundary near the largest diameter of the lamp vessel. A light source is arranged in the lamp vessel and current-supply conductors extend through the wall of the lamp vessel to the light source. Such a lamp is known from European Patent Specification No. 0 022 304 corresponding to abandoned U.S. Application Ser. No. 161,952.
Lamps of the kind described in the aforementioned European Patent Specification are manufactured by evaporating aluminum in the lamp vessel at a reduced pressure. For this purpose, a filament carrying a piece of aluminum is temporarily arranged in the lamp vessel. By current passage through this filament the aluminum is heated and evaporated. Unless this source of aluminum vapour is screened in part, substantially the whole lamp vessel is mirror-coated with a layer of aluminum.
Wall portions that should remain without a mirror-coating, can have their aluminum layer removed with lye. A sharp transition can then be obtained between wall portions that are mirror-coated and wall portions that are not mirror-coated. However, disadvantages of this manufacturing method are that the lye has to be completely removed by carefully washing the lamp vessel, that the lamp vessel has to be dried thoroughly, that the lye and the washing water used have to be made harmless for the environment and that there is a risk of the reflective layer being damaged by spatters of lye or washing-water.
Because of these disadvantages of the partial removal of a reflective coating, it is very attractive to be able to apply a reflective layer only at the areas at which it is desirable. The wall portion not to be coated could be covered with a mask. In most cases, however, this requires a mask which is larger than an opening in the lamp vessel (its neck), through which this mask has to be introduced. It has been suggested to use foldable masks which are expanded within the lamp vessel, but such masks are complicated and expensive. They have a short life because they soon cannot be fully expanded or folded any longer due to the fact that aluminum is deposited on them.
A simple and suitable method of partly mirror-coating a lamp vessel consists in that a screen is provided close to the vapour source, as a result of which a part of the wall of the lamp vessel lies in the shadow of this screen during evaporation of the aluminum. However, this method has the disadvantage that a part of the wall of the lamp vessel lies in a half-shadow. The lamp manufactured by this method has the disadvantage that a very thin aluminum layer has formed on the wall of the lamp vessel during evaporation at the area of the half-shadow. This very thin translucent aluminum layer manifests itself as a black zone which adjoins the mirror-coated wall portion near the largest diameter of the lamp vessel where the screen should have prevented deposition of aluminum.
The said half-shadow is caused by the fact that the vapor source is not infinitely small, but has a certain minimum volume when viewed by the surface to be covered. The half-shadow is also caused by the fact that aluminum vapor is exposed to the scattering effect of the residual gas in the lamp vessel on its way from the vapor source to the wall of the lamp vessel. The mirror-coating step is effected at reduced pressure, for example at 0.1 to 0.01 Pa, because an unacceptably long processing period would be involved in producing a high vacuum.
The dark zone limiting the mirror-coated wall portion in the known lamp is disadvantageous. The zone causes the lamp to have a poor aesthetic appearance and has an adverse effect on its appearance of quality. The zone does not reflect incident light from the light source efficiently, but does not transmit that light substantially completely either.